Protecting groundwater levels and ecosystems with simple management approaches

Abstract

Groundwater quantity is often managed using simple tools. The most common are (1) basin or sub-basin scale volumetric allocations, usually based on either historic use or estimates of recharge, (2) trigger-level management which regulates use according to observations of groundwater level, and (3) buffer zones, which control the location of wells, particularly around groundwater-dependent ecosystems (GDEs). The volumetric approach limits the long-term impact of abstraction and provides a stable, secure supply for groundwater users. However, this approach does not consider the spatial distribution of recharge and discharge, and so is poor at protecting GDEs. Buffer zones provide an effective means of limiting the short-term impact of abstraction on GDEs, and can also be used to shift impact from high to low priority GDEs. However, buffer zones mostly delay the impacts of abstraction on groundwater level and flow, and are less effective for managing long-term impacts. Groundwater response triggers aim to directly control groundwater levels, although the success of this approach is highly dependent on the location of the observation well, and the trigger value. This makes its successful implementation extremely difficult. Used alone, none of these approaches will successfully protect the environment. In combination, they can provide reasonable protection for ecosystems and reliability of groundwater supply for users.

Résumé

L’hydrogéologie quantitative est. souvent gérée à l’aide d’outils simples. Les plus courants sont (1) les allocations volumétriques à l’échelle du bassin ou du sous-bassin, basées généralement soit sur l’utilisation historique soit sur des estimations de la recharge, (2) la gestion d’un seuil d’alerte, qui régule l’utilisation en fonction des observations des niveaux piézométriques, et (3) les zones tampon, qui contrôlent l’emplacement des puits, en particulier autour des écosystèmes dépendant des eaux souterraines (EsDES). L’approche volumétrique limite l’impact à long terme de l’exploitation et fournit un approvisionnement stable et sécurisé pour les usagers des eaux souterraines. Cependant, cette approche ne prend pas en compte la distribution spatiale de la recharge et de la décharge, et de ce fait est. peu adaptée à la protection des EsDES. Les zones tampon fournissent des moyens efficaces de limitation de l’impact à court terme de l’exploitation sur les EsDES, et peuvent aussi être utilisées pour déplacer l’impact sur les EsDES d’une priorité élevée à moindre. Cependant, les zones tampon retardent principalement les impacts des prélèvements sur le niveau piézométrique et l’écoulement des eaux souterraines, et sont moins efficaces pour gérer les effets à long terme. Les seuils d’alerte à la réponse des eaux souterraines visent à contrôler directement les niveaux piézométriques, bien que le succès de cette approche dépende fortement de l’emplacement du piézomètre et de la valeur du seuil d’alerte. Ceci rend la réussite de sa mise en œuvre extrêmement difficile. Utilisée seule, aucune de ces approches ne protègera efficacement l’environnement. Combinées, elles peuvent assurer une protection acceptable des écosystèmes et la fiabilité de l’approvisionnement en eau souterraine pour les utilisateurs.

Resumen

La cantidad de agua subterránea a menudo se maneja con herramientas simples. Las más comunes son (1) asignaciones volumétricas de cuenca o subcuenca, generalmente basadas en el uso histórico o estimaciones de recarga, (2) nivel de activación de la gestión que regula el uso según las observaciones del nivel freático, y (3) zonas de amortiguamiento, que controlan la ubicación de los pozos, particularmente alrededor de los ecosistemas dependientes del agua subterránea (GDE). El enfoque volumétrico limita el impacto a largo plazo de la extracción y proporciona un suministro estable y seguro para los usuarios de aguas subterráneas. Sin embargo, este enfoque no tiene en cuenta la distribución espacial de la recarga y la descarga, por lo que es deficiente para proteger los GDE. Las zonas de amortiguamiento proporcionan un medio eficaz para limitar el impacto a corto plazo de la explotación en las GDE, y también se pueden usar para cambiar el impacto de las GDE de alta a baja prioridad. Sin embargo, principalmente las zonas de amortiguamiento retrasan los impactos de la extracción sobre el nivel y el caudal del agua subterránea, y son menos efectivas para la gestión de impactos a largo plazo. Los desencadenantes de respuesta de agua subterránea apuntan a controlar directamente los niveles de agua subterránea, aunque el éxito de este método depende en gran medida de la ubicación del pozo de observación y del valor de activación. Esto hace que su implementación exitosa sea extremadamente difícil. Utilizado solo, ninguno de estos enfoques protegerá con éxito el medio ambiente. En combinación, pueden proporcionar una protección razonable para los ecosistemas y la confiabilidad del suministro de agua subterránea para los usuarios.

摘要

经常采用简单的工具管理地下水量。最常见的是(1)流域或亚流域尺度的容积分配,通常基于历史利用状况或补给估算值;(2)根据地下水为观测结果调节利用量的触发水准管理;以及(3)控制井位的缓冲区,特别是在依赖于地下水的生态系统周围控制井位的缓冲区。容积方法消减了长期抽水的影响,并为地下水用户提供了稳定的、安全的供水。然而,这个方法没有考虑补给和排泄的空间分布,因此,在保护依赖于地下水的生态系统中表现很差。缓冲区提供了限制抽水对依赖于地下水系统的短期影响的有效方法,并可用来根据依赖于地下水系统的优先权的高低来改变影响程度。然而,缓冲区通常延迟了抽水对地下水位和水流的影响,在管理长期的影响中效果较差。地下水响应触发要素的目的就是直接控制地下水位,尽管此种方法成功高度依赖于观测井的位置和触发要素值。这就使该方法的成功实施变的非常困难。如果单单使用一种方法,这些方法没有一种能够成功保护环境。如果多种方法结合在一起,它们就能为生态系统提供合理的保护及为用户提供可靠的地下水供应。

Resumo

A quantidade disponível de águas subterrâneas é frequentemente gerenciada usando ferramentas simples. As mais comuns são (1) alocações volumétricas em escada de bacia ou sub-bacia de acordo com seu uso histórico ou estimativas de recarga, (2) gerenciamento por meio de valores gatilho para regular o uso de acordo com as observações do nível de água subterrânea, e (3) zonas de amortecimento, que controlam a alocação dos poços, particularmente em torno de ecossistemas dependentes de águas subterrâneas (EDASs). A abordagem volumétrica limita o impacto de longo prazo da abstração e fornece um suprimento estável e seguro para os usuários das águas subterrâneas. No entanto, esta abordagem não considera a distribuição espacial de recarga e descarga, e por isso é insuficiente para proteger EDASs. As zonas de amortecimento fornecem um meio eficaz de limitar o impacto de curto prazo nas EDASs, e também pode ser usado para mudar o impacto nas EDASs de alta para baixa prioridade. No entanto, as zonas de amortecimento atrasam, principalmente, os impactos da captação no nível e no fluxo das águas subterrâneas e são menos eficazes para o gerenciamento de impactos de longo prazo. Os valores gatilho de águas subterrâneas visam controlar diretamente os níveis de águas subterrâneas, embora o sucesso dessa abordagem seja altamente dependente da localização do poço de observação e do valor de referência. Isto torna extremamente difícil uma implementação bem sucedida. Usadas sozinhas, nenhuma dessas abordagens protegerá com sucesso o ambiente. Combinadas, eles podem fornecer proteção razoável para os ecossistemas e segurança hídrica de água subterrânea para os usuários.

Appendix

To investigate the impacts of the different management approaches, a numerical model was constructed using MODFLOW-NWT. The domain consisted of 1 layer (100 m thickness) with a cell size of 10 m × 10 m. The elliptical catchment is bounded by no flow boundaries with a constant head boundary on its left (Fig. 5). Uniform and isotropic hydraulic properties were used in the model: hydraulic conductivity of 1 m d⁻¹ and specific yield of 0.2. Uniform recharge of 50 mm yr⁻¹ was applied across the domain. The evapotranspiration (ET) package was used to simulate a terrestrial GDE near the centre of the catchment. This GDE had a maximum ET rate of 100 mm d⁻¹ when the groundwater level was less than 5 m from the surface, the ET rate reduced linearly to zero from 5 to 10 m below surface. Initial groundwater level and constant head boundary were 95 m from the base of the aquifer. The steady-state head distribution is shown in Fig. 5.

Fig. 5

Plan view of the model domain showing the steady-state groundwater levels (coloured contours). Boundary conditions are shown: no flow boundary (black line) and constant head boundary (blue line). The location of the terrestrial GDE is shown by the hashed circle, and the boundary of the buffer zone is indicated by the red circle. Eighteen wells (black and red dots) and one observation well (yellow dot) are shown. The three wells excluded from buffer zone simulations are shown in red

For the management scenarios, 18 wells were located within the catchment at various distances from the GDE. Four management scenarios where investigated using the model: (1) buffer zone approach, (2) combined buffer zone and volumetric approaches, (3) volumetric approach, and (4) a combined trigger level, buffer zone and volumetric approach. Abstraction rates for all scenarios followed the same structure; abstraction rates increase each year at the same rate (61,649 m³ yr⁻¹) until the specified abstraction rate or a groundwater threshold was reached. The total abstraction volume was equally distributed across the wells. For the volumetric allocation scenario, all 18 wells were used, and the maximum abstraction volume was limited to 40% of annual recharge volume. For the buffer zone scenario, a 500-m buffer was placed around the GDE which excluded three wells from the simulation (red circles in Fig. 5). The maximum abstraction rate for the buffer zone scenario was set to 120% of annual recharge. For the trigger level scenario, an observation point was located in the GDE (yellow circle in Fig. 5); this was monitored every 5 years. If the groundwater level at the observation well fell by more than 0.5 m compared to the steady-state level, the abstraction rate of the following 5 years was reduced by 50%. This reduced abstraction rate was maintained until the groundwater level in the observation well recovered (e.g., groundwater level less than 0.5 m below steady-state level).